The invention relates to the field of forming metal structures, and particularly to an apparatus for sealing tooling about vertically oriented, metal worksheets.
It is known that certain metals, such as titanium, and other alloys, exhibit superplasticity. Superplasticity is the capability of a material to develop unusually high tensile elongations with a reduced tendency towards necking. This capability is exhibited by only a limited number of metals and alloys, and within limited temperature and strain rate ranges.
Until the advent of viable superplastic forming techniques, taking advantage of this property to form complex configurations requiring large tensile elongations was extremely difficult, or in some instances, impossible. Simplified, the process involves placing a metal blank workpiece over a cavity in a die. The blank is heated to a temperature where it exhibits superplastic characteristics and a pressure differential is applied to the blank, causing it to stretch and form into the cavity. This process is disclosed in U.S. Pat. No. 3,934,441, "Controlled Environment Superplastic Forming of Metals", by Hamilton et. al. and is herein incorporated by reference.
Diffusion bonding refers to the metallurgical joining of surfaces of similar or dissimilar metals by applying heat and pressure for a sufficient time so as to cause commingling of the atoms at the joint interface. Diffusion bonding is accomplished entirely in the solid state, at or above one-half the base metal melting point. Actual times, temperatures and pressures will vary from metal to metal.
The combining of superplastic forming and diffusion bonding in the making of metallic sandwich structures has been successfully accomplished and is disclosed in U.S. Pat. No. 3,927,817, "Method of Making Metallic Sandwich Structures", by Hamilton, et. al., and is herein incorporated by reference.
Basically, the method for making metallic sandwich structures involves fabricating the structures from a stack of metal worksheets. Typically, the necessary tooling is incorporated within a hydraulic press. One or more of the sheets are coated in the selected areas not to be diffusion bonded. The sheets are positioned in a stacked relationship and are placed in a die assembly, wherein the stack is constrained at its periphery forming a seal thereabout. The sheets are diffusion bonded together in the uncoated areas by the controlled application of temperature and pressure, and at least one of the sheets is superplastically formed against one or more of the die surfaces, thereby forming the sandwich structure. The core configuration is determined by the location, size, and shape of the joined areas.
When the press is in continuous production, the metal structure is hot loaded and unloaded to avoid time consuming cooldown and reheat cycles. However, hot loading and unloading is extremely dangerous when performed manually from a press having horizontal rams. Also, removal of the hot part from a standard press often results in distortion of the part.
It is difficult to make a formed part within narrow tolerance, since the dies expand during forming temperatures. Also, the forming pressures within the die typically vary throughout the forming cycle, causing distortions by virtue of inappropriate die pressure. For example, die pressure which is too high can cause flow forming of the constrained periphery of the part, while die pressure which is too low can cause the seal to be lost or slippage of the preform during forming. Thus, in the past, to be sure that the structure remained in tolerance, excessive clamping forces were applied, aggravating the flow forming problem.
In the Hamilton sandwich process, metal worksheets are selectively coated with a stop-off material, and are placed in a stack within an enclosure. The stack is sealed within the enclosure and effectively constrained from further movement. The sheets are diffusion bonded together in the contact areas by applying compressive inert gas pressure. The stack is inflated and superplastically formed into the final structure.
U.S. Pat. No. 4,306,436 entitled "Method and Apparatus for Regulating Preselected Loads on Forming Dies" by D. W. Schulz, et. al., is also incorporated herein by reference. It discloses a horizontal press that uses mechanical pressure and a pressure bladder to form workpieces by superplastic forming or superplastic forming and diffusion bonding. The pressure bladder acts as a vernier or a find-adjustment for the mechanical pressure. However, the fatigue life of the pressure bladder is a problem, as the bladder must be replaced after relatively few cycles. The problem is compounded by the difficulty involved in replacing the bladders.
What is needed is a heavy duty, vertical forming press that will operate at elevated pressures and temperatures, and will overcome the disadvantages of the prior art.